I've thought about this and considered the following.
It should be perfectly possible to add something to a book someone else wrote, since it is possible in reality. However the addition from everyone but the original author would appear in a different, written font and its not possible to remove or change any of the original text. Maybe it could be possible to strike certain words through.
This change would both be more realistic and allows the creation of contracts, leaving notes, filling in names or adding signatures, etc.